UPDATED: Security Hack Exposes Forms Authentication in ASP.NET

Two security researchers, Thai Duong and Juliano Rizzo, have discovered a bug in the default encryption mechanism used to protect the cookies normally used to implement Forms Authentication in ASP.NET. Using their tool (the Padding Oracle Exploit Tool or POET), they can repeatedly modify an ASP.NET Forms Authentication cookie (normally encrypted using AES) and, by examining the errors returned, determine the Machine Key used to encrypt the cookie. The process is claimed to be 100 percent reliable and takes between 30 and 50 minutes for any site.

Once the Machine Key is determined, attackers can create bogus forms authentication cookies. If site designers have chosen the option to embed role information in the security cookie, then attackers could arbitrarily assign themselves to administrator roles. This exposure also affects other membership provider features, spoofing protection on the ViewState, and encrypted information that might be stored in cookies or otherwise be made available at the client. The attack will reportedly work on any block-cipher encryption mechanism (e.g. 3DES, MARS) implemented using the .NET encryption tools.

Microsoft is recommending, as a workaround, reducing information returned to the client in the event of an error to prevent intruders from gathering the information needed to determine the Machine Key. You can read more on that here. The simplest solution is, in the site's web.config file, to add or replace the customErrors tag inside the system.web element. The tag should point to an error page that provides no feedback on the error. A typical entry would look like this (where error.html is some html-only page in the same folder as the web.config file):

<customErrors mode="On" defaultRedirect="~/error.html" />

On a Web farm, these changes will have to be made on all the servers in the farm.

Peter Vogel is a principal in PH&V Information Services, specializing in Web development with expertise in SOA, client-side development, and user interface design. Peter tweets about his VSM columns with the hashtag #vogelarticles. His most recent book ("rtfm*") is on writing effective user manuals, and his blog posts on communicating effectively can be found at http://blog.learningtree.com/category/communication-2/.